blinkbiker87 wrote an answer about Arc'teryx Theta AR Jacket - Men's on August 12, 2010
Cillian, first of all this jacket is awesome. I have the Candy Apple Red and its pretty well the best looking jacket I have ever seen. Its light, durable and Im stoked to go alpine and ice climbing with it this winter. In terms of size, I think this jacket fits a bit big. This is emphasized by the AR denotation, meaning 'all around' so you can do whatever with it. Its meant to be layered with. I find I have to size down with arc for the 'slim' fit I like. I am about 6 feet and 160 and the small fits great with a t shirt but can still handle a down layer ect. I would go with the XS because I think you would be swimming in the small.
Hope this helps!